You will destroy resale, looks, and usability of the boat all at once. Most will listen to reason and do the right thing. Some will spend a lot on a "better" idea which has been proven not to work in the past. I think putting an outboard on this stern to try and push a 13,500 pound boat at any controllable speed won't work and I think others will agree. A vertical transom would be more workable but still a bad idea. Smaller outboards aren't really designed to push this weight either. A better choice would be to either fix the inboard or swap it for a rebuilt inboard. If you do this work yourself it probably won't cost much more than what you plan and it will work. But it's your money so it's your bad choice to make.
Living aboard in Victoria Harbour